1. Field of the Invention
The present invention relates to a monitoring system, a monitoring method, and an imaging apparatus used in a surveillance camera or the like.
2. Description of the Related Art
Generally, monitoring systems are used for surveillance over a wide area, for example, for sea surveillance, river surveillance, monitoring of restricted areas, observation of the behavior of wild animals, etc. In monitoring systems, video cameras having an extremely large number of pixels are used in order to shoot an image of a wide area. Accordingly, there is a problem in that the systems are expensive and high costs are incurred. In order to solve this problem, a method has been proposed in which many still images are shot while successively shifting a shooting area of a camera and an image of a monitored area is obtained by connecting the still images. In this case, an image having an extremely high resolution can be obtained as the overall image, and when an enlarged view of a part of the overall image is required, a clear, high-resolution image can be obtained as the enlarged view.
In the case of shooting still images while successively shifting the shooting area of the camera, it is necessary to prevent an imaging unit from moving while an object image is being acquired. More specifically, it is necessary to stop the imaging unit each time the object image is acquired.
As shown in FIG. 14A, a camera 301 includes a lens unit 302 and an imaging unit 303. The camera 301 is moved around the center of a reference circle 304 in the horizontal plane, and the optical axis of the camera 301 is defined as a reference line 305. The lens unit 302 includes a plurality of lenses.
As shown in FIG. 14B, the camera 301 is moved stepwise in the horizontal plane and still images are successively shot at positions indicated by (1), (2), and (3).
More specifically, a still image of an object at the position indicated by (1) is acquired by the imaging unit 303 while the movement of the camera 301 is stopped, and after the image is acquired, the camera 301 is moved to the next shooting area, that is, the shooting area at the position indicated by (2). When the camera 301 reaches the shooting area at the position indicated by (2), the movement of the camera 301 is stopped and a still image of an object at the position indicated by (2) is acquired by the imaging unit 303. After the image is acquired, the camera 301 is moved to the shooting area at the position indicated by (3). When the camera 301 reaches the shooting area at the position indicated by (3), the movement of the camera 301 is stopped and a still image of an object at the position indicated by (3) is acquired by the imaging unit 303. After the image is acquired, the camera 301 is moved to the next shooting area. The above-described processes are repeated to obtain many still images.
In this system, it is strongly required to reduce the time for shooting the still images. However, according to known techniques, in order to reduce the time cycle at which the still images are acquired, it is necessary to reduce the time for taking each still image and increase the moving speed of the camera.
When the time for taking each of the still images is reduced, that is, when a shutter speed is increased, although an image corresponding to an instantaneous angle of view can be obtained, sufficient amount of light cannot be obtained when the shutter speed exceeds a certain limit. When the amount of light is insufficient, only dark and dull images can be obtained.
In addition, when the moving speed of the camera is increased, the shutter speed must be necessarily increased, and the above-described problem occurs in that sufficient amount of light cannot be obtained when the speed exceeds a certain limit. In addition, since the camera must be quickly moved to a desired position and then be stopped, there is also a problem in that high-accuracy motors are necessary and high costs are incurred.
In addition, another method is known in which an optical path along which an object image travels is changed in accordance with the movement of the camera so that the object image incident on the imaging unit does not change even while the camera is being moved. In this case, the lens unit of the camera includes an optical-path changing element which changes the optical path. When the camera moves, the displacement of the camera is detected by an acceleration sensor or an angular-velocity sensor, and the optical-path changing element is controlled on the basis of the detected displacement so that the object image incident on the imaging unit does not change.
However, the acceleration sensor or the angular velocity sensor and a feedback circuit are necessary to control the optical-path changing element, and cumbersome design work is required.